1. Field of the Invention
This invention generally relates to a lock-up control system for a torque converter for an automatic transmission.
2. Description of the Prior Art
A torque converter generally includes a pump impeller connected with an engine output shaft to be driven thereby, a turbine runner having an output member and a stator disposed between the pump impeller and the turbine runner, so that hydraulic fluid is circulated by the engine driven pump impeller through the turbine runner with the aid of the stator which functions to deflect the hydraulic fluid from the turbine runner in a direction in which the fluid flow does not disturb the toration of the pump impeller when the hydraulic fluid flows into the pump impeller. With the circulation of the hydraulic fluid in this way, the turbine runner is subjected to a hydrodynamic force to be driven thereby. The torque converter has an automatic speed changing function wherein the torque is amplified when the speed of the turbine runner is lower than that of the pump impeller and the output torque gradually decreases as the speed of the turbine runner approaches to that of the pump impeller. However, the torque converter is disadvantageous in that it inherently has slippage between the pump impeller and the turbine runner which causes a decrease in transmission efficiency and poor fuel economy.
It has therefore been proposed to provide a lock-up clutch mechanism between the input and output shafts of the torque converter and have the lock-up clutch engaged when the turbine runner speed is close to the pump impeller speed so that the turbine runner is rotated as a unit with the pump impeller. For example, in Japanese Unexamined Patent Publication No. 56(1981)-138559, there is disclosed a torque converter lock-up control system which has a memory map provided with a lock-up control line defining a lock-up zone and a lock-up release zone as a function of the vehicle speed and the engine load. That is, in the lock-up control system, whether or not the operating condition is in the lock-up zone is determined from the engine speed represented by a signal from a vehicle speed detector which detects the vehicle speed from the rpm of the output shaft of the transmission and the engine load represented by a signal from an engine load detector which detects the engine load from the negative pressure in the intake passage. When the point having detected values of the vehicle speed and the engine load as its ordered pair (on a coordinate plane the abscissa and the ordinate of which respectively represent the vehicle speed and the engine load) falls in the lock-up zone on the higher vehicle speed side of said lock-up control line, the lock-up mechanism is actuated to effect lock-up, while when the point falls on the other side of the lock-up control line, the operation of the lock-up mechanism is interrupted to release the lock-up.
This lock-up control system is considered to be advantageous in that the torque converter lock-up is automatically controlled in a preferable manner according to the operating condition of the engine, thereby obtaining improved fuel economy. However, it should be noted that in the lock-up control system, there is a possibility that the lock-up mechanism may be engaged even when the throttle valve is fully closed if the operating condition of the engine is in the lock-up zone. This causes instable operation of the engine since the engine output torque may fluctuate when the throttle valve is fully closed so that vibration or instable engine operation may be produced if the torque converter is locked up.
In Japanese Unexamined Patent Publication No. 56(1981)-39353, there is disclosed a lock-up control system for a torque converter in which the lock-up mechanism is released near or at the full close position of the throttle valve irrespective of the relationship between the operating condition of the engine and the lock-up control line. However, this system is not satisfactory in the following three respects.